DYNAMICAL PROPERTIES OF SUPERFLUID TURBULENCE

Despite all the experimental work done in recent years to study superfluid turbulence, our understanding of the dynamical properties of this system is still poor. We have designed a new cryogenic probe to perform a series of experiments to study the dynamical response of the vortex line density in turbulent thermal counterflow.;These experiments have established the existence of fluctuations in the chemical potential in turbulent counterflow. For the first time fluctuations in the dissipation were observed in the TI/TII transition region. The fluctuations are characteristic of broad-band noise showing no evidence of fluctuations at preferred frequency. The TI/TII transition is characterized by a sharp increase in the noise power amplitude and its variation with heat current reveals a complex structure. Some of that structure is shown to correspond with features observed in the steady state value of the dissipation. A narrow peak in power amplitude can be associated with a sharp increase in the exponential response time at the same heat current. This effect similar to "critical slowing down" also corresponds to a sharp increase in the steady state value of the line density.;The apparatus uses a small glass flow tube that let us probe the fluctuations in the line density around the two turbulent states (TI,TII) present in this system. We use a chemical potential gradiometer that measures the chemical potential across the flow tube. This quantity is directly related to the vortex line density. The gradiometer also let us study both the steady state and the dynamical properties of the turbulence.